The invention relates to a locking device for a magnetic tape cassette having engagements in its underside. The cassette is received in an insertion slot of a magnetic tape device. A lock-in device holds the magnetic tape cassette in a reference position and an ejection means is also provided.
When sensing a magnetic tape with a magnetic head in a magnetic tape device, it is necessary for disruption-free operation that the magnetic tape and the sensing head be disposed in a defined position relative to one another. For that purpose, a plurality of reference stops are provided. The number and position is specified, for example, in the ECMA Standard No. 46. However, there is the danger that the magnetic tape cassette will change its position relative to the reference stops, or relative to the sensing head during sensing if it is not sufficiently secured.
For that purpose a lock-in device is known which exhibits two spring-tensioned rollers which constantly extend from below into the insertion slot. When a magnetic tape cassette is inserted, these rollers engage with the engagements and thus hold the cassette fast. It is a disadvantage, however, that a relatively great force is required in order to introduce or, respectively, withdraw the magnetic tape cassette into or, respectively, from the insertion slot, since the spring power of the rollers must be overcome. Also, the magnetic tape cassette is only pressed against two of the reference stops, so that only a limited lock-in is achieved. Since, moreover, it is already pressed against the two reference stops during introduction or, respectively, withdrawal, it is subject to high wear at the points of contact.
These disadvantages are avoided by a further, known lock-in device which exhibits three spring-tension arms provided with rollers and a motor. The motor drives two cam plates which hold two of the arms outside of the insertion slot until a cassette has been inserted and a position sensor has emitted a control signal for lock-in or, respectively, when an operator has given a control signal for unlocking. During that time, the third arm is held outside of the insertion slot by means of a spring connected to the two first arms. The lock-in ensues such that, by means of turning the cam plates, the blockage of the two first arms is released so that they are pressed into the insertion slot and against the underside of the magnetic tape cassette by their spring tension. At the same time, the cam plate likewise presses the third arm into the insertion slot and against the magnetic tape cassette. What is disadvantageous is the large space requirement arising due to the employment of the motor. Moreover, the assembly of the large number of individual parts requires considerable time. The danger is present in both lock-in devices that the magnetic tape cassette will release from the reference position when it is influenced by an external force which neutralizes the spring forces.